The present invention relates to a structure of an electronic part which may be mounted on and soldered to the surface of a printed circuit board, and a method of mounting such an electronic part.
It is a common practice in the electronic devices art to mount electronic parts and circuits on a single-layer or laminated printed circuit board to thereby enhance miniaturization. Especially, a current trend is toward electronic parts of the type being mountable on the surface of a printed circuit board without resorting to holes or similar means. A high-frequency power amplifying module is one of such surface-mountable electronic parts included in a portable or on-board mobile radio communication apparatus. Since a power amplifying module for this kind of application generates heat while in operation, a case thereof for housing circuitry is usually provided with a flat heat radiating plate made of metal. The heat radiating plate contacts a printed circuit board over the entire area of one major surface thereof which serves as a ground terminal. A gallium arsenide transistor is available as a power amplifying module and has various advantages such as low heat generation, high power conversion efficiency, and low power consumption.
However, conventional power amplifying modules have some problems left unsolved since they have a flat metallic heat radiating plate without exception. Specifically, since the heat radiating plate has substantial heat capacity, a great amount of heat is needed in soldering the plate to a printed circuit board as a ground electrode. Firm bonding, therefore, is not achievable with ordinary soldering methods which rely on dipping, reflow, laser beam, etc. While a special soldering arrangement capable of generating a greater amount of heat may be used, it is likely that the heat is imparted to the circuitry accommodated in the case and other electronic parts mounted on the circuit board to damage them. Further, since the heat radiating plate contacts the circuit board over the entire surface thereof, it cannot sufficiently radiate heat generated by the module to the ambience. This heats module itself and the circuit board on which the module is mounted and thereby critically deteriorates the module and circuit board as well as other electronic parts mounted on the circuit board.
Connecting terminals extending out from the power amplifying module and connected to the circuitry are soldered to the circuit board in the same manner as the heat radiating plate. For this purpose, a pattern provided on the circuit board has lands to which the connecting terminals are to be soldered. Each connecting terminal is usually implemented as a flat metallic piece and, therefore, needs a great amount of heat for soldering and increases the soldering time. Flux is apt to form bubbles between the surfaces of the flat connecting terminals and those of the lands of the pattern during soldering, reducing the bonding strength between them. Further, when the module is heavy, the weak bonding strength would prevent it from withstanding a shock ascribable to a fall, for example.
Regarding the reflow method which is one of conventional soldering methods, it can solder all the necessary electronic parts to the lands of the pattern provided on the circuit board, but the required soldering condition differs from a relatively large part to a relatively small chip. For this reason, it has been customary to solder the individual electronic parts one by one to the circuit board by reflow at the cost of efficiency. Soldering using a laser beam is successful in eliminating this problem. However, when a relatively large electronic part is to be soldered to the circuit board by a laser beam, chips neighboring the connecting terminals of the part of interest have to be prevented from being irradiated. This cannot be done unless the laser beam is turned on and off every time a connecting terminal is soldered, also resulting in inefficient operations.